Ternary Diagrams are used for a wide variety of applications in the Earth Sciences, including illustrating rock classification schemes and depicting chemical compositions on certain types of phase diagrams. It is essential that you understand how to use these plots!

This module will examine these plots in depth, and provide a short exercise for testing your understanding of the material!

Ternary diagrams are used to represent the relative percentage of three components. These components can be anything... three minerals, three elements, three size classes, etc. The only requirement is that the three components have to sum to 100%. (Yes, that means if they don't, you have to normalize them to 100%.)

This is a ternary diagram, where any point represents the relative percentage of three components, A, B and C.

Notice there are no numbers on the axes. This is done deliberately to emphasize the point that each axis intersection actually represents the intersection of two percentage values as shown in the blow up. Along each axes, there are two components represented and one is equal to zero.

The "composition" of any point plotted on a ternary diagram can be determined (or any point can be plotted) by following the steps illustrated in the image below. Percentage values for A are read from zero along the basal line (axis) at the bottom of the diagram to 100% at the vertex of the triangle. Similarly, values for B are read from zero along the upper left axis, to 100% at the lower right vertex. And finally, values for C are read from zero along the upper right axis to 100% at the lower left vertex.

Simple? Straightforward? Lets test your understanding!

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